This invention relates to an idling rpm feedback control method for internal combustion engines, which can eliminate a time lag in the control of the supply of fuel to the engine when a sudden change occurs in the quantity of supplementary air being supplied to the engine.
An internal combustion engine can easily stall due to a drop in the engine speed when the engine is operated in an idling condition at a low temperature of the engine cooling water or when the engine is heavily loaded by head lamps, air conditioner, etc. in a vehicle equipped with the engine. Also, on such occasions, unburnt ingredients such as CO and HC are emitted from the engine, badly affecting the emission characteristics and fuel consumption.
To eliminate such disadvantages, an idling rpm feedback control method has been proposed, e.g. by Japanese Patent Provisional Publication (Kokai) No. 55-98628 or U.S. Pat. No. 4,289,100 issued on Sept. 15, 1981 in the names of Kinugawa et al., which comprises setting desired idling rpm in dependence upon load on the engine, detecting the difference between actual engine rpm and desired idling rpm, and supplying supplementary air to the engine in a quantity corresponding to the detected difference so as to minimize the same difference, to thereby control the engine rpm to the desired idling rpm.
It has also been proposed by Japanese Patent provisional Publication (Kokai) No. 58-197449, or the above U.S. Pat. No. 4,289,100 to detect on- and off-states of electrical devices or equipments such as the headlamps, the brake lamp, the radiator cooling fan, and mechanical devices or equipments directly driven by the engine, such as the air conditioner and the automatic transmission during the above-mentioned idling rpm feedback control, and upon detecting a transition in the operative state of such an equipment, varying the quantity of supplementary air by a predetermined amount corresponding to the magnitude of the electrical load or the mechanical load applied by the device concerned, thereby minimizing lag of the engine rpm control.
According to this proposed method, when a control valve for controlling the supplementary air quantity is opened upon detection of a transition to an on-state of one of the electrical devices or the mechanical devices, and for a period of time corresponding to a required increase in the supplementary air quantity, there is a time lag between the opening of the control valve and the arrival of the increased supplementary air to the engine cylinders. Such time lag is determined by the configuration and size of the supplementary air passage extending from the control valve to the engine cylinders.
Further, an electronic fuel supply control system is already known which detects the total quantity of intake air inclusive of supplementary air being supplied to the engine and controls the fuel supply quantity in response to the resulting detected value, so as to achieve a desired air-fuel ratio of the mixture. However, even with this proposal, due to a lag in the detection of a change in the intake air quantity, which is attributed to the characteristics of the intake air quantity sensor, the mixture becomes lean instantaneously when there is a rapid increase in the supplementary air quantity, causing engine stall or hunting of the engine rotation, whereas the mixture becomes too rich when there is a rapid decrease in the supplementary air quantity, not only deteriorating the emission characteristics but also causing hunting of the engine rotation to create discomfort to the driver.